White Pine Blister Rust in Juvenile Western White Pine on State Lands in Washington
Daniel Omdal and Melanie Kallas Ricklefs
Symptoms
Introduction
Western white pine (Pinus monticola Dougl.) was once an integral part of the forest ecosystems of Washington.
Due to its ability to tolerate extremes in site conditions, it was historically found interspersed throughout most
vegetation zones. Around 1910, Cronartium ribicola J.C. Fisch., the causal organism of white pine blister rust
(WPBR), was introduced into western North America on infected nursery stock grown in Europe and shipped
to British Columbia. From British Columbia, the disease spread to Washington causing widespread mortality
throughout the range of five needle (white) pines. White pine blister rust has become the most destructive
disease of white pines in North America.
Branch flagging is the most obvious symptom of WPBR (fig. 3). When the fungus encircles the branch, it girdles
and kills the branch rapidly, resulting in flagging. Girdling of the main stem results in topkill (fig. 4). Retention of
red needles is most common in recently killed branches and treetops. Cankers on young, smooth-barked trees start
out as an orange discoloration in the bark with or without swelling. As the canker expands in size, it will often
have a rough center surrounded by a diamond-shaped orange lesion of infected bark (fig. 5). The rough center is
the portion of the canker that has sporulated, while the orange discoloration of the bark represents the leading edge
of the fungus.
Over the last two decades, the USDA Forest Service and University of Idaho have established breeding
programs to genetically enhance western white pine for resistance to white pine blister rust. During this time,
the Washington Department of Natural Resources (DNR) has been steadily increasing the outplanting of
western white pine seedlings on state lands. In 2001, more than 200,000 genetically enhanced western white
pine seedlings were planted on state lands in Washington. Nevertheless, white pine blister rust remains a
component of the forest ecosystems throughout the Pacific Northwest. A 2002 survey of western white pine
saplings and mature trees in Washington revealed infection levels of up to 100% in some geographic regions. It
is unknown what percentage of the white pine surveyed in this study was genetically enhanced, however, it is
unlikely that western white pine planted on state lands west of the Cascade Mountains prior to 2001 was
genetically enhanced. Due to high white pine blister rust infection levels, much, if not all of the western white
pine planted on state lands since 2001 is genetically enhanced stock.
High levels of white pine blister rust in western white pine saplings suggest that mortality due to white pine
blister rust may be underestimated by excluding juvenile white pine from these studies. It is likely that some of
the planted western white pine succumb to blister rust prior to reaching the sapling stage. In fact, recent
investigations of juvenile western white pine stands (less than five years old) revealed high levels of infection in
some geographical regions of Washington. However, the majority of sites assessed had been planted with stock
that was not genetically enhanced for white pine blister rust resistance.
High infection levels in unimproved juvenile western white pine indicate that, in some regions, juvenile western
white pine may suffer significant infection and mortality prior to reaching the sapling stage. It is believed that
the genetically enhanced stock would experience less infection and mortality than unimproved stock. The focus
of this study is to monitor the incidence and severity of WPBR infection, and thus determine the success of the
resistance mechanisms, in genetically enhanced juvenile western white pine.
The Pathogen
Cronartium ribicola is a fungus that infects all 5-needle pines and requires Ribes spp. (gooseberries and
currants) as alternate hosts to complete its life cycle. The fungus infects pine through the needles and grows
down the twigs into the branches and ultimately into the main stem. In the spring, aecial blisters (fig. 1)
break through the bark of infected tissue giving cankers a blistered appearance. The aecial blisters produce
spores that infect only Ribes spp. Once on a Ribes plant, the fungus initially produces spores that re-infect
only Ribes spp. (fig. 2) and may amplify available inoculum. In the fall, spores that infect pine are produced
on Ribes spp.
Figure 3. Branch flagging on
western white pine sapling.
Figure 4. Topkill of western
white pine sapling.
Figure 5. Lesion on smooth bark of
western white pine sapling.
Methods
• Two recently planted (2001 or later) management units containing a significant amount of genetically
enhanced juvenile western white pine will be selected from each geographic region of Washington state
using the Washington DNR Forest Resource Inventory System.
• Permanent plots consisting of 100 western white pine seedlings will be established in each of these
management units.
• The seedlings will be visually assessed for WPBR cankers.
• The number and location of WPBR cankers, along with any other information regarding seedling
health, will be recorded.
• Mortality will be recorded during plot establishment, but only live seedlings will be tagged for use in the
permanent plot.
• Plots will be assessed for WPBR each year, for a total of three years.
Objectives
There are no current surveys that assess the incidence and severity of white pine blister rust on genetically
enhanced western white pine in Washington. Therefore, we have no data indicating whether or not the genetic
enhancement improves the chances of survival in the field. Survival of juvenile western white pine is critical to
restoring the species in forest ecosystems of Washington. While western white pine is not listed as a species of
concern, five-needle pines are a key species in many Pacific Northwest ecosystems and they have been
significantly impacted over the last century. Restoration of western white pine is consistent with the goals of the
Northwest Forest Plan to maintain and restore the natural diversity and health of the northwest forest ecosystem.
This study will help us to quantitatively describe the relative success, over time, of genetically enhanced western
white pine in resisting infection and mortality caused by white pine blister rust.
Contact Information:
Daniel Omdal
dan.omdal@wadnr.gov
Melanie Kallas Ricklefs
melanie.kallas@wadnr.gov
Washington Department of Natural Resources
P.O. Box 47037
Olympia, WA 98504-7037
Figure 1. Aecial blisters on
juvenile western white pine.
Figure 2. Infected Ribes spp. leaf.
(360) 902-1692
(360) 902-1395
Acknowledgements:
This project is being funded by an Evaluation Monitoring grant from the USDA Forest Service’s Forest Health Monitoring
program.